The invention relates to a rear-projection viewing screen of the type comprised of a lens-aperture matrix and a cooperating lens matrix, the apertures of the lens-aperture matrix corresponding in size, position and spacing to the size, position and spacing of the lens-matrix elements.
The basic optical principle of this type of rear-projection viewing screen is as follows: The apertures of the lens-aperture matrix lie in approximately the common focal plane of the elements of the lens matrix. The incident light received by each lens-matrix element is focussed at and projected through the associated aperture of the lens-aperture matrix. Accordingly, substantially all the light incident upon the back side of the viewing screen emerges from the front side of the viewing screen. However, because of the focussed passage of such light through the apertures of the lens-aperture matrix, the intensity of the light passing through each such aperture will be very considerably increased, compared to the intensity of the light incident upon the associated lens-matrix element. On the other hand, the image formed on the front side of the viewing screen is a dot image, necessarily exhibiting a reduced resolution. However, the loss of resolution associated with the dot image is more than offset by the boosted intensity of the light dots. Particularly if the viewing screen is viewed in a brightly lit room, the boosted intensity of the light dots of the dot image makes, on the whole, for an increased viewability or legibility of the image on the viewing screen. Additionally, the surface of the lens-aperture matrix surrounding the apertures thereof absorbs the ambient light, and may even be painted a dull black to maximize this absorption. This results in a rear-projection viewing screen of the "black surround" type; i.e., each light dot of boosted light intensity is surrounded by a black background which promotes a psychological impression of even further increased image light intensity, as a result of the strong background contrast.
An important disadvantage of known rear-projection viewing screens of this general type is that the marginal portions of the image appearing on the screen cannot be seen by the eye of a viewer if his eye lies for example on the optical axis of the objective of the rear-projection projector. The viewer can see all the marginal portions of the screen image only by moving his head in such a manner as to bring his eye within the viewing ranges associated with the marginal portions of the viewing screen.
This is depicted in FIG. 7a. It will be seen that the light emanating from the image dots near the marginal portion of the viewing screen does not reach a viewer's eye located on the optical axis of the projector objective.
The elimination of this problem in the case of conventional lens matrixes for microfilm viewing screens, for example, would require lens matrixes having relative apertures of 1:1.3, which can scarcely be realized in practice.
For this reason, it is known to insert a layer of light-scattering material behind viewing screens of the "black surround" type. This is shown in FIG. 7b. The scattering of the light emanating from the marginal portions of the viewing screen does cause some of such light to reach a viewer's eye located on the optical axis of the projector objective. Accordingly, the marginal portions of the screen image are at least faintly visible. However, the visibility is only faint. Moreover, the uniformity of the illumination of the screen achieved in this fashion is considerably inferior to what could be achieved using the same layer of light-scattering material in the conventional way as the viewing screen itself, without any lens matrix. The reason for this is that the scattering layer, due to the apertures, is illuminated only in discrete almost point-shaped regions, which in practice has an effect similar to a corresponding coarsening of the scattering layer structure.